Pictured is an artist's impression of the two forms of ultra-viscous liquid water with different densities. In the background is depicted the X-ray speckle pattern taken from actual data of high-density amorphous ice, which is produced by pressurizing water at very low temperatures. |
Mattias Karlén

Materials

Water May Exist as Two Very Different Liquids Simultaneously

Scientists have long pondered the strange qualities of water, and researchers in Sweden have just found that at room temperature it might at once exist as two distinct liquids with differing densities and molecular structures.

Water may seem simple enough. But it’s actually a bizarre substance with outlandish chemical properties that combine to make life on Earth possible as we know it.

Consider just one of them: Unlike most other liquids, water becomes less dense when it solidifies. That means the world’s oceans freeze only on the surface, enabling life to develop and thrive within Earth’s vast liquid oceans despite turbulent and frosty ice ages.

All told, water has about 70 unusual physical properties covering variables like melting point, density, and heat storage.

Now, researchers in Sweden have added a startling new twist to water’s complex biography by proving that at low temperatures, water actually exists as two distinct liquids with differing densities and molecular structures.

“Water is very unusual, it behaves very strangely,” said Anders Nilsson, professor in chemical physics at Stockholm University. “Water can, indeed, at very low temperatures, exist as two distinct liquids.”

By probing cold water with X-rays, Nilsson and his colleagues observed that when cooled to a temperature just above freezing, water was found to exist in two separate phases: one in which the molecules are packed more tightly together, and one in which they are more loosely arranged.

The discovery, published this week in the journal Proceedings of the National Academy of Sciences, presents fresh evidence for a view advocated by Nilsson, but which remains unproven and which has been hotly debated for years. The theory argues that water, at room temperature and at the mean atmospheric pressure at sea level, exists in a constant state of flux between two different types of liquids as infinitesimally small volumes fluctuate at a rate measured in picoseconds (or trillionths of a second).

If this view is accurate, then a glass of water sitting on a kitchen table could be seen as a pint-sized sea of high density water holding tiny islands of low-density water that appear and disappear at the speed of once per trillionth of a second.

If ultimately confirmed, this unusual dual nature could help explain some of water’s more uncommon characteristics, Nilsson said, including water’s high capacity to store heat, and the fact that water achieves maximum density at four degrees Celsius.

“The new results give very strong support to a picture where water at room temperature can’t decide in which of the two forms it should be, high or low density, which results in local fluctuations between the two,” said one of Nilsson’s collaborators, Lars G.M. Pettersson, professor in theoretical chemical physics at Stockholm University. “In a nutshell: Water is not a complicated liquid, but two simple liquids with a complicated relationship.”

While this new discovery may not yield immediate applications in the real world, Nilsson points out that a stronger understanding of water’s weird properties could help lead to more powerful purification and desalination technology — which could one day expand access to fresh water, an increasingly vital and endangered resource amid global population growth and climate change.

“We need to understand these things better because one of the biggest challenges for humanity going forward is going to be access to clean water,” Nilsson said. “And in order to go further, we need to have a fundamental understanding of water.”